The present invention relates to capacitive devices, and more particularly, to a programmable capacitor in an integrated circuit.
In integrated circuits (IC""s), capacitors are commonly used for data storage, signal filtering, and timing adjustments. However, conventional IC capacitors are difficult and/or costly to produce within an IC due to the nature of current wafer processing techniques. FIG. 1A shows a conventional planar capacitor 101, which comprises a polysilicon layer 130 and an oxide layer 140 formed on a p-type substrate 110. Polysilicon layer 130 and a depletion region 112 in substrate 110 provide an upper plate and a lower plate, respectively, for planar capacitor 101. The dielectric constant of oxide layer 140, along with the area of polysilicon layer 130 and the area of depletion region 112 control the capacitance of capacitor 101. The simple geometry of planar capacitor 101 is relatively straightforward to manufacture. However, the planar construction of planar capacitor 101 requires that polysilicon layer 130 occupy a large area on the surface of an IC die. This large area makes the construction of planar capacitor 101 increasingly problematic as IC die sizes decrease and device densities increase.
FIG. 1B shows a conventional trench capacitor 102, comprising a polysilicon layer 132 having a plate portion 133, and an oxide layer 142 extending into a trench 114 formed in a substrate 110. Oxide layer 142 provides a dielectric layer between plate portion 133 and a depletion region 113 formed in substrate 110 around trench 114. By orienting the capacitor xe2x80x9cplatesxe2x80x9d in the vertical direction, trench capacitor 102 occupies significantly less IC die surface area than planar capacitor 101. However, the irregular geometry of trench capacitor 102 significantly increases manufacturing complexity, thereby leading to increased cost and decreased reliability.
FIG. 1C shows a conventional stacked capacitor 103, comprising an oxide layer 144 sandwiched by an upper polysilicon layer 134 and an intermediate polysilicon layer 150. Stacked capacitor 103 is formed over an NMOS transistor 160. NMOS transistor 160 is not an essential component of stacked capacitor 103, and can be replaced with other IC structures, such as bipolar transistors or resistive elements. NMOS transistor 160 comprises a polysilicon gate 162 and a gate oxide 164 formed over two n-type regions 120 in substrate 110. An oxide layer 166 provides a surface insulating layer for NMOS transistor 160. Intermediate polysilicon layer 150 is deposited over one of the n-type regions 120 of NMOS transistor 160 and a portion of oxide layer 166. Intermediate polysilicon layer 150 also extends over a field oxide 124 that isolates NMOS transistor 160 from adjacent IC devices. Oxide layer 144 is formed over intermediate polysilicon layer 150, and upper polysilicon layer 134 is deposited over oxide layer 144 to complete stacked capacitor 103. The non-planar contours of upper polysilicon layer 134 and intermediate polysilicon layer 150 increase their effective surface areas, thereby increasing the capacitance of stacked capacitor 103. Because stacked capacitor 103 is xe2x80x9cstackedxe2x80x9d over an existing IC structure, efficient IC die surface area utilization is provided. At the same time, the deep etch and subsequent step coverage issues of trench capacitor 102 are avoided. However, while stacked capacitor 103 is easier to produce than trench capacitor 102, the formation of intermediate polysilicon layer 150 requires an additional polysilicon deposition step, thereby increasing overall manufacturing cost and cycle time for an IC including stacked capacitor 103.
Due to variations inherent in semiconductor manufacturing processes, specific capacitance values are difficult to produce using the aforementioned conventional capacitance structures. The dielectric constant of an oxide layer can vary between production runs, and precise control of oxide layer thickness is difficult to achieve. Also, the non-planar configurations of the trench and stacked capacitors makes the areas of the polysilicon xe2x80x9cplatesxe2x80x9d difficult to accurately control. Finally, during normal IC operation, temperature effects can change the material properties of the capacitive structures, leading to further variations in actual capacitance values. Therefore, conventional capacitive structures are ill-suited for situations requiring precise capacitance settings, such as delay lines and bandpass filters.
Accordingly, it is desirable to provide a capacitive structure in an IC that is compact, easily manufacturable, controllable, and adaptable to process and operating variations.
The present invention is directed towards apparatus and methods for creating capacitance in an integrated circuit (IC), overcoming the cost and accuracy limitations of conventional capacitive structures by utilizing controlled parasitic capacitance effects.
Typically, IC manufacturers attempt to eliminate parasitic (unwanted) capacitances in IC""s. An IC comprises a variety of functional devices configured to perform specified sets of tasks. Dielectric material separates and isolates the functional devices from one another. Interconnects provide conductive paths between functional devices, thereby allowing signals to be transmitted from one functional device to another. When interconnects run parallel to one another, parasitic capacitances can be generated which impose undesirable effects on the signals travelling along the interconnects. Maintaining a large spacing between interconnects alleviates the problem, but the industry trend towards shrinking IC die sizes and increasing device densities makes such a technique unfeasible. Alternative methods such as multilevel, orthogonal placement of interconnects can reduce the effects of parasitic capacitances, but also increase manufacturing cost and complexity.
In an embodiment of the present invention, a conductive line is coupled to a bias control circuit. The conductive line is positioned parallel to an interconnect that electrically connects two IC devices within an integrated circuit. The bias control circuit applies a bias voltage to the parallel conductive line to induce a parasitic capacitance between the interconnect and the parallel conductive line. By making the parasitic capacitance equal to a desired capacitance, signals transmitted between the two IC devices along the interconnect can be delayed or filtered. The magnitude of the parasitic capacitance is controlled by the length of the parallel conductive line, the distance between the interconnect and the parallel conductive line, and the dielectric constant of the material between the interconnect and the parallel conductive line. Unlike an interconnect, which provides a conductive path between two or more IC devices, the parallel conductive line has no specific routing requirements, and can therefore be sized as necessary to provide the desired capacitance.
In another embodiment of the present invention, the bias control circuit comprises logic circuits to selectively bias the parallel conductive line to a desired voltage potential. Removal of the bias voltage from the parallel conductive line minimizes the capacitive path between the interconnect and the conductive line, thereby allowing the parasitic capacitance to be decoupled from the interconnect as desired.
In another embodiment of the present invention, multiple parallel conductive lines are placed along side the interconnect. By changing the number of the multiple parallel conductive lines to which the bias voltage is applied, the total capacitance coupled to the interconnect can be adjusted.